Some things to consider..
1. Are you reducing the "HT" multiplier to keep the overall HT speed below the 1300 range?
2. Have you set a RAM divider so that your RAM isn't hitting an unstable speed?
Try a default of DDR667 or 333MHz, that will under clock the RAM to about 83% of the CPU speed and allow the RAM to increase at that same % to the CPU bus OC.

Do take a look in the AMD/RAM section Sticky on DDR2 Dividers for more insight, if needed